1. Field of the Invention
The present invention relates to a resistor element suitable for absorbing the surge accompanying the opening/closing of, for example, a breaker, particularly, to an electrical resistor element suitable for use in a power equipment such as a breaker and a method of manufacturing the same.
2. Description of the Related Art
Various electrical resistor elements are widely used nowadays. For example, such resistor elements are used for the current control in breakers, etc., for various controls accompanying the starting and stopping of motors, and for the grounding in the occurrence of abnormalities in the power transmitting system. The resistor bodies used in these resistor elements include, for example, metallic resistors, ceramic resistors and various composite resistors.
In, for example, a breaker for a high voltage, an closing resistor is connected in parallel with a breaking point in order to absorb the surge accompanying the opening/closing of the breaker and to increase the breaking capacitance. A resistor body of a resistor element used for such an object is disclosed in, for example, Published Unexamined Japanese Patent Application No. 58-139401. In this prior art, used is a ceramic resistor body having carbon particles dispersed therein. To be more specific, the conductive component of carbon powder is dispersed in the insulating component of aluminum oxide crystals, and the resultant body is fired using clay so as to manufacture the resistor body disclosed in this prior art. It is taught that the resistor body has a resistivity of 100 to 2500 .OMEGA..multidot.cm.
It is certainly possible to control the resistivity of the resistor body by adjusting the amount of the carbon powder dispersed in the aluminum oxide crystals. However, the resistor body of this type has such a high porosity as 10% to 30%. In other words, the resistor body is low in its density, giving rise to various problems. First of all, the heat capacity per unit volume is as small as 2 J/cc.multidot.K, leading to a small resistance to discharge. As a result, a marked temperature elevation is brought about by the heat generation accompanying the surge absorption. What should also be noted is that a discharge takes place among the carbon particles in the stage of absorbing the surge accompanying the opening/closing of the breaker, leading to a discharge throughout the resistor element. In short, the conventional resistor body disclosed in Published Unexamined Japanese Patent Application No. 58-139401 leaves room for further improvement when used as a resistor body in a breaker for a high voltage.
Further, in accordance with miniaturization of the breaker achieved by the marked technical progress in recent years, it is required to miniaturize the input resistor element for absorbing the surge accompanying the opening/closing of the breaker. In order to miniaturize the closing resistor element, it is necessary for the resistor body included in the resistor element to have a large heat capacity per unit volume. It is difficult to further miniaturize the closing resistor element by using the conventional resistor body having a heat capacity of 2 J/cm3.multidot.K.
The difficulty noted above can be overcome by using a ceramic resistor such as ferrite which has such a large heat capacity per unit volume as 3.0 to 4.0 J/cm3.multidot.K.
In general, ferrite is represented by a chemical formula Me.sub.x Fe.sub.3-x O.sub.4, which is one of spinel type oxides. The metal "Me" included in the formula given above may be Ni, Co, Zn, Cu, etc. In general, one of these metals is contained in the ferrite. However, it is possible for Me to denote a plurality of these metals contained together in the ferrite. Also, the value of "x" in the formula given above is generally 1. However, "x" falls within a range of between 0 and 3 in some kinds of ferrite. In this case, the resistivity of the ferrite can be controlled by suitably selecting the value of "x". For example, FIG. 1 shows that the resistivity of cobalt ferrite (Co.sub.x Fe.sub.3-x O.sub.4) can be changed within a range of between 10.sup.2 .OMEGA..multidot.cm and 10.sup.8 .OMEGA..multidot.cm by suitably selecting the value of "x" (See, for example, J. Phys. Chem. Solids 9, 165 (1959)).
However, some components of the ferrite are partly evaporated depending on the sintering temperature and composition, leading to a change in the composition of the ferrite. As a result, it is impossible to obtain a large sintered body, for example, a disk-like sintered body having a diameter of 10 cm or more because of the problems noted below. First of all, the evaporation noted above makes it difficult to obtain a dense sintered body. A second problem is that, in the case of manufacturing a plurality of sintered bodies, the resultant sintered bodies are made nonuniform in resistivity. What should also be noted is that the evaporation takes place nonuniformly within the sintered body, with the result that the sintered body comprises a high resistivity portion and a low resistivity portion. In other words, a resistivity distribution is formed in the sintered body. Under the circumstances, where a resistor element including a resistor body formed of ferrite is used for absorbing the surge accompanying the opening/closing of a breaker heat is generated in the high resistivity portion of the resistor body. As a result, the resistor body tends to be broken by the thermal stress caused by the heat generation.
The oxide semiconductor such as ferrite gives rise an additional difficulty. Specifically, the resistivity is lowered with increase in the temperature. The temperature coefficient of resistivity (TCR), which denotes the change with temperature in resistivity, is defined as follows in view of the conditions under which the input resistor element for the surge absorption is used: ##EQU1##
where .rho..sub.120 the resistivity at 120.degree. C. and .rho..sub.20 represents the resistivity at 20.degree. C. For example, a cobalt ferrite sintered body sintered under the air atmosphere has a temperature coefficient of resistivity of about -0.70 to -0.99%/deg. In other words, if the temperature of the resistor body is elevated by 100.degree. C. from 20.degree. C. to 120.degree. C., the resistivity at 120.degree. C. is lowered by 70% to 99% from the resistivity at 20.degree. C. to reach 30% to 1% of the value at 20.degree. C. It follows that, if the temperature of the resistor body is elevated by the surge absorption, the resistivity of the resistor body is lowered so as to bring about a thermal runaway. In this case, it is impossible for the resistor element to absorb completely the surge accompanying the opening/closing of the breaker. In order to prevent the thermal runaway, it is desirable for the resistor body of the closing resistor element to have a temperature coefficient of resistivity of at least -0.30%/deg, preferably at least -0.20%/deg.
To reiterate, ferrite is widely known to the art in respect of the electrical properties such as the resistivity and the thermal properties such as the heat capacity. However, a resistor body effective for the surge absorption has not yet been developed.